Methods for conveying fine-grained powders are required, for example, for conveying dosed quantities of fine-grained powders for plasma coating processes. Deposits and agglomerations of the powder in the conveying paths and the pump must he avoided, since otherwise the powder conveyance can come to a standstill.
The conveyance of fine powders with a particle size of less than 150 μm is scarcely possible with known pumps. Below this grain size, the adhesion forces between the powder particles increase considerably. The surface area of the particles relative to their volume increases steeply. A cube with an edge length of 1 cm has a surface of 0.006 m2. However, the same volume filled with particles of five nanometers edge length has a surface area of 2400 m2. The steep increase in the surface adhesion forces impairs the conveyance of such small particles. By continuously coupling energy into the powder, for example, by sustaining high flow speeds, which is associated to a high gas or air consumption, agglomeration of the powder/gas mixture can be avoided. High gas volume flows are, however, disadvantageous in many subsequent working processes, such as, for example, in plasma-coating processes or laser-coating processes. Furthermore, high gas volume streams require higher energy application for the powder conveyance.
German Patent Application No. 44 23 197 A1 discloses a powder pump for the spray-coating of articles in a bar-type elongated shape. On a front side, the powder pump has a powder inlet opening, via which the powder is aspirated from an upwardly open powder container. The powder is subsequently conveyed via an inner tube of the powder pump to a consumer. The conveyance itself is driven by generating a vacuum within the powder pump. The vacuum is generated with an injector nozzle arranged in the vicinity of the powder inlet opening.
Furthermore, diaphragm pumps for the conveyance of gases and fluids are known from prior art. The working space is separated by a deflectable membrane from the pump drive. By virtue of this separation, the pump drive is shielded from harmful effects incurred from the conveyed medium. During a suction cycle, the oscillating deflection of the membrane increases the working volume of the conveyance chamber while fully deflected in the suction position, and likewise reduces the working volume of the conveyance chamber white the fully deflected in the pressure position. The deflection of the membrane is driven hydraulically, pneumatically, or mechanically. On the suction side of the diaphragm pump, an inlet valve is arranged that is actuated by the medium conveyed. On the pressure side, an outlet valve is arranged that is likewise actuated by the medium conveyed. During the intake stroke of the membrane, the conveyed medium is aspirated via the inlet valve. During the compression stroke of the membrane, the conveyed medium is exhaled via the outlet valve.
When using a diaphragm pump for the conveyance of a powder/gas mixture, for example, containing fine-grained powders, the flow speed in the working volume of the conveyance chamber of the diaphragm pump is typically insufficient to exhale the entire amount of powder through the outlet valve.
Consequently, increasing accumulations of powder form inside the conveyance chamber of the diaphragm pump as the operation of the diaphragm pump goes on, thus, reducing the suction power and eventually blocking the diaphragm pump. For example, for very fine-grained powders, powder agglomerates can form that Obstruct the powder conveyance or block the diaphragm pump. A blockage of the diaphragm pump cannot be avoided by increasing the pumping power. Instead, it is necessary to open the conveyance chamber and to remove the obstruction.
Another influence factor on the formation of deposits is the geometric shape of the conveyance chamber. Powder deposits form, for example, in regions with a lower flow velocity of the powder/gas mixture. It has also been found that blockages occur in the region of the outlet valves during the conveyance of fine-grained powders. Mitigation of these ‘problem zones’ in the conveyance chamber by increasing the flow rate of a diaphragm pump with a given working range, from a particular negative pressure on the suction side and a particular pressure at the pressure side, is not readily possible.